1. Field of the Invention
The present invention relates to an electrostatic image forming method, and, more particularly, to a method of forming an electrostatic image on the insulating surface portions which are formed in a pattern-like manner on an electrostatic image forming material together with the conductive surface portions.
2. Description of the Prior Art
In the electrostatic printing field, charging the insulating surface portions of a material which has conductive surface portions in addition to the insulating surface portions on an electrostatic image forming material has been widely accomplished. In this conventional method, more specifically, the electrostatic image forming material, on which the conductive and insulating surface portions are distributed in a pattern-like manner, is placed on a conductive base plate. Then, charges are applied to the insulating portions, for example, using a corona charging process, with the conductive portions being grounded to the earth, thus froming the desired electrostatic image.
Where an electrostatic image forming material which has a conductive intermediate layer formed on an insulating support and insulating layer portions formed on the conductive intermediate layer in a pattern-like manner is used in the conventional method, it is easy to ground the conductive layer to the earth with satisfactory results.
On the other hand, another method has been proposed in which an image forming material which has conductive surface portions and insulating surface portions both distributed in a pattern-like manner is used, thus dispensing with the conductive intermediate layer on the insulating support.
In accordance with the above method, a method has been proposed in which a conductive layer capable of being peeled off is formed on the surface of an insulating support. Desired portions of the conductive layer are then removed in a pattern-like manner using a stencil pen or the like so as to form recessed and exposed insulating surface portions on the surface of the insulating support. Charges are applied to the recessed insulating surface portions using a corona charging process or the like to thereby form a desired electrostatic image thereon.
In this method, moreover, when the electrostatic image on the insulating surface portions is intended to be transferred by applying toners thereto, the toner image obtained is formed on the recessed surface portions, so that the toner image will scarcely be damaged even if registration in the transfer operation is carried out. Therefore, this method is quite important in obtaining satisfactory transfer quality.
However, the method, in which the corona charging process is applied to the electrostatic image forming material formed with the conductive and insulating surface portions without the formation of the conductive intermediate layer on the insulating surface, is inevitably accompanied by the following drawbacks.
Reference will now be made to FIGS. 1 to 4 of the accompanying drawings. In FIG. 1a, reference numeral 1 indicates an electrostatic image forming material (which will be referred to hereinafter, for brevity, as a "material"), which is composed of conductive surface portions 4 in which at least the surfaces are conductive and formed on an insulating support 2 and of insulating surface portions 3 formed in a pattern-like manner on the insulating support 2. The insulating surface portions 3 are formed in such a fashion that a conductive layer 4, which is formed on the surface of the insulating support 2 to constitute the conductive surface portions 4, is removed in a pattern-like manner mechanically or chemically so a to expose the desired surface portions to the outside.
For illustrative purposes only, the insulating surface portions 3 are shown in FIG. 1b to have a shape of a modified letter Q. In this case where the letter Q is engraved, the insulating surface portions 3 are divided into the following portions:
a. The portion 3' which is surrounded by conductive layer portion 4" around the letter;
b. The portion 3" which is surrounded both by the conductive layer portion 4" and by island-shaped conductive layer portions 4' electrically isolated from the portion 4"; and
c. The portion 3"' which is surrounded both by the portion 4" and by the portions 4'.
The "material" 1 as above is placed on a conductive base plate 5, which is grounded to the earth as shown in FIG. 1a, with its back face down. A corona charging process is then carried out by moving above the material a corona charger 6, which is connected with a power source 7 and whose corona electrode 6' is impressed with a DC high voltage in the direction of the arrow. If the conductive layer portion 4" around the letter Q is grounded to the earth in the manner as shown in FIGS. 1a and 1b, the charging operation is effected by the following actions.
At an initial stage of the charging operation, the corona ions are substantially uniformly applied to the surface of the material. When the charging operation proceeds, the electric field of the material will reach the condition as shown in FIG. 2. In FIG. 2, the broken line arrows will indicate the behavior of the corona ions irradiated from the electrode 6' of the corona charger 6.
Among the ions thus uniformly applied, the ions applied to the surrounding conductive layer portion 4" will be neutralized because the portion 4" is grounded to the earth, whereas the ions applied to the insulating surface portions 3 and the island layer portions 4' will be stored therein as shown by the plus signs. The ions thus stored will exhibit a blocking action to succeeding corona ions, which are approaching to be applied to the material as shown by the broken line arrows. These charges establishing the blocking or repulsing electric field will be referred to for brevity as "blocking charges". Especially, the ions, which are approaching the vicinity of the insulating surface portions 3' and 3" having both of its sides or one of its side grounded to the earth, will be subjected to the repulsive force of the ions having the blocking charges and accordingly will charge their courses such that they will be caught by the conductive layer portion 4" and neutralized.
On the contrary, the ions, which are approaching both the conductive island layer portions 4' electrically isolated from the surrounding layer portion 4" and the insulating surface portion 3"' surrounded by the portions 4', will be further trapped therein because they are separated from the grounded conductive layer portion 4" and accordingly because they cannot leak thereto. The trapping or storing action of the charges will continue until they build up sufficiently as blocking charges against the corona ions coming, or until an equibirium condition is attained with the potential of the electrode 6' of the corona charger 6. As a result, the charges stored in the conductive island layer portions 4' and in the insulating surface portion 3"' surrounded thereby will be increased to raise the potentials of the portions 4' and 3"'. On the other hand, as these potentials become higher and higher increasing their repulsive forces, it becomes more and more difficult for corona charges to approach the insulating suface portion 3", which are defined both by the conductive island layer portions 4' and by the grounded conductive layer portion 4".
The charges thus obtained are illustrated in FIG. 3 in terms of the distribution of the surface potentials.
In FIG. 3, the abscissa is taken from the A - A' cross-section of FIG. 1b and the ordinate indicates the surface potentials of the respective portions. As shown, the potential in the grounded conductive layer portion 4" surrounding the letter Q is zero, and the potentials in the conductive island layer portions 4' isolated from the layer portion 4" have a certain level because the charges impinging upon the layer portions 4' are stored therein and cannot leak to the layer portion 4".
That portion 3' of the insulating surface portions 3 forming the image, which is surrounded by the grounded conductive layer portion 4", has a cetatin potential at its center, but this potential decreases in value toward the periphery until it becomes zero at its peripheral edges. This is because, during the charging operation, the ions leak to the conductive layer portion 4" which is grounded to the earth. On the other hand, in the insulating surface portion 3" which is defined by the conductive island layer portions 4' and by the grounded conductive layer portion 4", the coming corona ions are repulsed by the blocking voltage of the conductive island portions 4', so that the surface portions 3" have a potential of a certain level at the periphery of the island portions 4' but a potential which decreases toward the periphery of the grounded conductive layer portion 4" until it becomes zero at the peripheral edge of the portion 4". On the contrary, the insulating surface portion 3"', which is surrounded by the conductive island layer portions 4', has a uniform potential of a certain level.
As shown in FIG. 3, the level of the uniform potential of the insulating surface portion 3"' is much higher than the maximum level of the insulating surface portion 3', which is surrounded by the conductive layer portion 4". This is confirmed by the experimental results, as shown in FIG. 4. FIG. 4 is a graphical presentation, in which the surface potentials both of the insulating surface portion 3' surrounded by the grounded conductive layer portion 4" and of the insulating surface portion 3"' defined by the conductive island layer portions 4' are plotted against the impressed voltage. The potential of the insulating surface portion 3' is, as can be understood from this graph, saturated at a certain level. On the contrary, the potential of the insulating surface portion 3"' defined by the conductive island layer portions 4' is increased continuously with an increase in the impressed voltage of the corona charging operation.
As is apparent from the foregoing descriptions, the following disadvantages will arise, in the case where an electrostatic image is formed in the insulating surface portions of an electrostatic image forming material which has electrically isolated conductive layer portions on its surface.
First of all, the electrostatic image obtained has a limited low potential. This means that a visual image of high density cannot be obtained after the electrostatic image is developed;
Secondly, the distribution of the charges in the insulating surface portions forming the electrostatic image cannot be made uniform. This phenomenon arises because the potential at the peripheral edge of the insulating surface portion adjoining the conductive layer portion will have a low level. After having been subjected to a developing treatment, the electrostatic image thus obtained cannot produce a clear visual image of high contrast.
Thirdly, the potential of the insulating surface portion will have different levels depending upon whether the conductive layer portions surrounding the insulating surface portion are grounded or not. The electrostatic image thus obtained will produce a non-uniformity in the visual image.
The third disadvantage can be obviated by grounding all of the island portions to the earth, but this grounding operation becomes more difficult as the number of islands increases. As a matter of fact, however, it is almost impossible to ground all of the islands because an actual electrostatic image is composed of so many islands. Even if all of the islands could be grounded, moreover, the first and second disadvantages still remain.